Phase evolution in lithium disilicate glass–ceramics based on non-stoichiometric compositions of a multi-component system: structural studies by 29Si single and double resonance solid state NMR

The crystallization mechanism of a high-strength lithium disilicate glass–ceramic in the SiO2–Li2O–P2O5–Al2O3–K2O–(ZrO2) system, used as restorative dentistry material, has been examined on the basis of quantitative 29Si magic angle spinning (MAS) and 29Si{7Li} rotational echo double resonance (REDOR) NMR spectroscopy. Crystallization occurs in two stages: near 650 °C a significant fraction of the Q(3) units disproportionates into crystalline Li2SiO3 and Q(4) units. Upon further annealing of this glass–ceramic to 850 °C the crystalline Li2SiO3 phase reacts with the Q(4) units of the softened residual glass matrix, resulting in the crystallization of Li2Si2O5. The NMR experiments provide detailed insight into the spatial distribution of the lithium ions suggesting the absence of lithium ion clustering in the residual glassy component of the final glass–ceramic. 31P MAS-NMR spectra indicate that phosphate acts as a lithium ion scavenger, resulting in the predominant formation of orthophosphate (P(0)) and some pyrophosphate (P(1)) groups. Crystallization of Li2SiO3 occurs concomitantly with the formation of a highly disordered Li3PO4 phase as evidenced from strong linebroadening effects in the 31P MAS-NMR spectra. Well-crystallized Li3PO4 is only formed at annealing conditions resulting in the formation of crystalline lithium disilicate. These results argue against an epitaxial nucleation process previously proposed in the literature and rather suggest that the nucleation of both lithium metasilicate and lithium disilicate starts at the phase boundary between the disordered lithium phosphate phase and the glass matrix.